why distortion and what is the cure

The answer would be easy if you were using a turntable: there is a point where the SPL in the room is causing the TT to go into feedback. The solution is to separate/isolate the TT from the speakers.
BUT, if you are using a CD as the source, then there are (too)many possible answers.

Thanks Joe
I am using CD as the source. The sound with both amps is fine at low volume but turn it up and yuck!! Have spoken to Sugden who suggest that A)one or two of the power transistors may have gone or B)the speakers arent sensitive enough for the amp. Amp is 12 watts class A and speakers are as above.I love the sound of the amp and really dont want to change it if i can help it.The speakers can go out with the trash. So cmon guys - solutions and speaker suggestions (pref F/S with waf)

I'm guessing its not a vibration issue, but simply overdriving your poor 12 W amp. You don't exactly have efficient speakers, so if you are trying to listen at high volumes (1 o'clock on my pre is deafening), your Sugden is probably running out of gas, distorting and clipping.

I'd have to agree with Edesilva's last comment, it sounds as if you have hit the output limits of your system. Many systems are at or near peak output at the 1-2 o'clock positions. And tube sound typically goes downhill very fast once you hit peak output.